For many years seismic exploration for oil and gas has involved the use of a source of seismic energy and its reception by an array of seismic detectors, generally referred to as geophones. When used on land, the source of seismic energy can be a high explosive charge electrically detonated in a borehole located at a selected point on the terrain, or some other energy source having capacity for delivering a series of impacts or mechanical vibrations to the earth's surface. The acoustic waves generated in the earth by these sources are partially reflected from various earth layers and transmitted back from layer boundaries and reach the surface of the earth at varying intervals of time, depending on the distance and characteristics of the subsurface traversed. These returning waves are detected by the geophones, which function to transduce such acoustic waves into representative electrical signals. In use an arrangement of geophones is generally laid out along a line to form a series of observation stations within a desired locality. The source injects an acoustic signal into the earth, and the detected signals, which are reflected from points midway between the source and receiver are recorded for later processing. These recorded signals, which are continuous electrical analog signals depicting amplitude versus time, are generally quantized using digital computers so that each data sample point may be operated on individually. The geophone arrangement is then moved along the line to a new position where some of the shot or receiver points may overlap, and the process repeated to provide a seismic survey. If the ground and subsurface reflecting layer are flat, as previously mentioned a seismic shot yields data from midway between the source and receiver. One of the techniques utilized in processing seismic data is to combine traces produced from two or more shots wherein the midpoint between the source and the receiver in each case is the same, although the offset, i.e., sources to receiver distance, may be different. When two or more traces belonging to a common midpoint (CMP) are summed, the technique is called common-midpoint stacking.
A single wave-producing activation of a source, called a shot, results in generating a number of traces equal to the number of receivers. Aligning all of the recorded traces from a single shot in a side by side display i.e. shot gather can produce a rudimentary two dimensional seismic section. The section can be improved, however, by the CMP stacking. Since sound traveling two different paths gives information from approximately the same subsurface point, two such traces reflected from a common point can be combined, i.e., summed, such that reflection amplitudes are added but the noise, which occurs at different times on the two traces, is not added thus improving the signal-to-noise ratio. The number of traces summed in an individual stack is called the multifold or simply the fold.
More recently, seismic surveys involve geophones and sources laid out in more complex geometries, generally involving rectangular grids covering an area of interest so as to expand areal coverage and enable construction of three-dimensional (3-D) views of reflector positions over wide areas. A normal prior art three-dimensional survey geometry is shown in FIG. 1, in which a basic grid, indicated generally at 21, is defined for effective placement of shotpoints that are designated as squares 24, and geophone receivers that are designated as crosses 22. As illustrated, the basic grid 21 is a square having a dimension d.sub.1 that is equal to twice the desired reflection midpoint spacing, and that will provide an image having a desired resolution of subsurface features. A plurality of geophone receiver lines 20.sub.a -20.sub.n each containing a plurality of equally spaced apart geophone receivers 22 is place in parallel on the earth's surface. A plurality of shotpoints 24 is placed along source or shot lines 26.sub.a -26.sub.n which run orthogonally to the receiver lines 20.sub.a -20.sub.n, thus providing a symmetrical crossed array geometry with geophone receivers 22 in lines 20.sub.a -20.sub.n and source stations 24 in lines 26.sub.a -26.sub.n spaced apart a distance equal to d.sub.1, and the lines 26.sub.a-n and 26.sub.a-n spaced apart a distance of four times d.sub.1. This crossed-arrayed geometry produces subsurface spatial resolution in which midpoints are spaced apart by one-half of the distance d.sub.1 in the receiver line, and one-half of the distance d.sub.1 in the source line. For example, if receivers and sources, as shown in FIG. 1, are each spaced 165 ft. apart, reflection midpoints will be spaced apart by 82.5 ft. and four adjacent midpoints will form a square.
It is well known, however, to those skilled in the art that improved surface sampling resolution in a survey can be obtained with a source/receiver geometry that is referred to herein as "true 3-D coverage." This geometry also uses CMP stacking in which the shotpoints and receivers are laid out in the generally rectangular areas similar to the arrangement shown in FIG. 1, but with closer spacing of the receiver lines 20 in the shotpoint line direction. As used herein, a true 3-D seismic source/receiver geometry locates a geophone receiver and/or a shotpoint at each intersection of the basic grid 21. An example of true 3-D seismic source/receiver geometry having a geophone receiver at each intersection of the basic grid 21, and having shotpoints spaced apart at a distance four times d.sub.1 is illustrated in FIG. 2. Using the same size basic grid 21 as shown in FIG. 1, such a true 3-D layout would include 400 geophone receivers and 25 shotpoints covering a surface area 19 in FIG. 2(a) that is equal to the surface area 19 shown in FIG. 1. Once all of the receivers and shotpoints are in place, the shots are sequentially activated and a number of traces that is equal to the number of shots times the number of receivers are recorded to provide a single data set from which a display of a seismic 2-D section, or a 3-D volume could be produced. In this true 3-D technique the recorded traces having common midoints, which are sorted out later from the recorded traces, are gathered in a display which yield greatly increased surface resolution compared to the surface resolution shown in FIG. 1.
In seismic acquisition and processing operations, it is well known to those skilled in the art that a frequency ambiguity called aliasing is inherent in sampling systems, and that aliasing occurs in a sampling process when there are fewer than two samples per cycle. Aliasing applies to both the time and space domains. The aliasing that is done by the separated elements of geophone receivers and shotpoints is called spatial aliasing and depends on the surface spacing of the shotpoints and receiver. The aliasing that is done by sampling an input signal is called frequency aliasing and is dependent on the sampling interval used to digitize input signals. To avoid aliasing filtering is commonly required. For example, an alias filter applied before sampling a geophone signal at a ground location removes certain undesired frequencies, likewise a velocity filter of a seismic gather attenuates certain coherent arrivals of waves, which sweep over the geophone receivers having certain apparent receiver velocities. Accordingly, an advantage of true 3-D seismic source/receiver layout geometry is avoiding spatial aliasing.
There is a disadvantage to this kind of true 3-D shooting, however, in the excessive amount of equipment required to occupy every surface location with a receiver and/or a source on a grid interval equal to twice the desired subsurface resolution. Today, however, a normal 3-D seismic survey based on a layout geometry similar to FIG. 1 is an accepted part of the early data-acquisition process because the high resolution display of 3-D surveys leads to an optimized appraisal of sites, refined reserve estimates, and more efficient development plans. Accordingly, the benefits of a 3-D survey using source receiver geometry similar to that shown in FIG. 1, although having certain known deficiencies, usually outweigh the additional cost compared to a 2-D survey.
Accordingly, if use of 3D seismic surveys is to continue to grow, a need exists for new and improved methods that simplify and/or provide economical alternatives that reduce the operational cost of obtaining a 3-D seismic survey.
It is an object of the present invention to provide flexibility in use of given equipment for field operations that obtain 3-D seismic survey data.
It is a more specific object of the invention to gather partial data from a section of a survey area having source line and receiver line resolution that facilitates unaliased 3-D digital filtering.
Another more specific object is to provide an early look at subsurface features from partial prestacked reconnaissance data having characteristics similar to 3-D processed data, so as to guide further planning of a seismic survey program, and/or processing of the seismic data.